1. Technical Field
This invention relates generally to developer mix monitors for electrostatographic machines, and more particularly to such machines capable of developing a series of electrostatic images with different toners, for example, different color toners.
2. Background Art
U.S. Pat. No. 4,928,146 issued to Yamada on May 22, 1990, is illustrative of a number of references which show the development of a series of electrostatic images carried on a photoconductive drum with different colored toners at a single development position. See also, U.S. Pat. No. 3,797,930, Tanaka et al, issued Mar. 17, 1974; U.S. Pat. No. 4,275,134, Knechtel, issued June 23, 1981; Japanese Kokai 1-244477 (1989); U.S. Pat. No. 4,728,983, Zwaldo, issued Mar. 1, 1988. A series of four development units are moved one after another to the development position. Each unit develops an image and is replaced by another unit as the series of units is indexed to apply a different color toner to the next image. The series of units are arranged side-by-side and moved linearly through a position in which the unit to be used is aligned with the development position. After or as it is aligned, a cam is rotated to push the entire unit toward the development position, generally moving transverse to the motion of the series of units. Other references show the units arranged on a rotary support with the units being rotated through alignment with the development position.
This general approach has the advantage of utilizing only a single development position for applying four different color toners to electrostatic images. This permits the use of development units whose size and number would prohibit them being spaced around the periphery of a relatively small photoconductive drum. It thus also permits the use of a small photoconductive drum. The use of a small drum has many advantages including both reduced expense and reduced size of the apparatus. U.S. patent application Ser. No. 07/621,681, filed in the names of Kroll et al on Dec. 3, 1990, relates to such apparatus.
Electrostatographic machines generally uses a two-component developer mix comprised of a toner powder and a magnetizable carrier material. During the use of the apparatus, toner powder has to be replenished in a quantity necessary to compensate for the consumption of toner powder used for the development of images. Various automatic toner replenishment systems are known wherein a signal representative of the detected concentration of toner powder in the developer mix is compared with a reference signal corresponding to a predetermined reference toner concentration. When the detected concentration is lower than the reference concentration, replenishment occurs.
U.S. Pat. No. 4,620,783 discloses a color copier having a plurality of development units indexable into alignment with a photoconductive drum. A single toner monitor is provided for ascertaining the ration of toner particles to carrier particles in the development mixture of the particular development unit aligned with the drum. No provision is made for compensation for long-term variables (noise) in the monitored signal, such as noise due to the thermal characteristics of the monitor, contamination, mechanical misalignment of structural parts, pressure changes between the monitor and the toning station walls, etc.